1. Field of the Invention
The present invention relates to a radar system including a radar device for transmitting and receiving a radio wave to and from a target to be detected.
2. Description of the Related Art
In recent years, a standard for a bus, such as CAN (Control Area Network), for use on a vehicle such as a car, has been proposed, and there is a need for a radar device installed on a vehicle and connected to such a bus. In a case in which various devices, in addition to a radar device, are connected to such a general purpose bus, the relative period of time during which the radar exclusively uses the bus must be as short as possible. For example, because of a limited bandwidth of the bus, it is not possible to directly transmit data acquired by scanning the beam over an entire bearing range via the bus.
To solve this problem with the limited bandwidth of a bus connected to a radar device, techniques of reducing the data size transmitted via the bus are disclosed in the following patent documents: (1) Japanese Unexamined Patent Application Publication No. 9-133765; (2) Japanese Unexamined Patent Application Publication No. 3-251781; (3) Japanese Unexamined Patent Application Publication No. 3-295485; and (4) Japanese Unexamined Patent Application Publication No. 7-35844.
The radar device disclosed in patent document (1) is an FM-CW radar device in which after an analog beat signal is converted into digital form and is passed through a digital filter, the resultant data is partially removed.
In the radar device disclosed in patent document (2), a radar video signal obtained by performing sampling at varying intervals is read at regular intervals thereby compressing the data size of the radar video signal.
In the radar device disclosed in patent document (3), a predetermined pattern is subtracted from an original signal and then resultant data is divided according to threshold values and multiplexed.
In the radar device disclosed in patent document (4), data is divided in accordance with a threshold value that is equal to the mean value of time-varying data plus an offset value, and clutter data having a value that is lower than the threshold value is compressed along the time axis.
However, in the radar device disclosed in patent document (1), high-frequency components of the beat signal are lost. As a result, information associated with targets at distant locations is not obtained. In the radar device disclosed in patent document (2) degradation in resolution occurs in a range (distance range) in which sampling is performed at large intervals. In the radar devices disclosed in (3) and (4), data compression is performed only along the time axis (corresponding to the distance), and thus high compression ratios are not obtained.
In order to overcome the problems described above, preferred embodiments of the present invention provide a radar system that is capable of transmitting necessary information using a very small amount of data from a radar device to a host device via a bus with a limited bandwidth.
According to a preferred embodiment of the present invention, a radar system includes a radar device and a host device wherein the radar device includes a transmitter for transmitting a frequency-modulated transmission signal and generating a beat signal between a portion of the transmission signal and a signal reflected from a target, a spectrum determining unit for determining a spectrum associated with the signal strength of the beat signal, a data compression unit for compressing the spectrum data by removing redundancy from the spectrum data, and a transmitter for transmitting the compressed data via a transmission line, and the host device includes a receiver for receiving the compressed data via the transmission line, decompressing the compressed data, and processing the decompressed data to detect the target.
The radar device may further include a scanner for scanning a beam of the transmission signal within a predetermined range of bearing angle, and the data compression unit may determine the difference in spectrum of the beat signal between adjacent bearing angles.
The data compression unit may also determine the difference in signal strength between spectral components at adjacent locations on the frequency axis.
The radar device may further include a scanner for scanning the beam of the transmission signal within the predetermined range of bearing angle repeatedly from one frame to another, and the data compression unit may determine the difference in spectrum of the beat signal between different frames at the same beam bearing angle.
The coding is preferably performed after replacing data, whose original signal strength is lower than a first threshold value and whose difference value is lower than a second threshold value, with a specific value such as zero.
In addition, the radar device may further include a determining unit for determining detection data associated with a target on the basis of the spectrum, and a transmitter for selectively transmitting either the detection data or the compressed data to the host device via the transmission line.
Data compression is preferably performed by removing redundancy from the spectrum data, and the resultant compressed data is transmitted to the host device via a transmission line such as a bus line. This makes it possible to transmit data necessary to detect a target to the host device via a transmission line with a narrow bandwidth without losing information.
Furthermore, the difference in spectrum of the beat signal between adjacent bearing angles is preferably determined, or the difference in spectrum of the beat signal between adjacent locations on the frequency axis is preferably determined. In the above-described process of determining the difference, it is necessary to temporarily store only spectrum data at one beam bearing angle at a time. Thus, a high capacity of memory is not necessary in the data compression process. This allows a reduction in cost of the radar system. Furthermore, no reduction in frequency resolution occurs.
The difference in spectrum between frames which are adjacent to each other in the time axis is preferably determined, wherein the frame is a unit of data obtained by scanning the beam once over the scanning range. This makes it possible to compress, in a highly efficient fashion, data associated with a target in the same beam bearing angle along the time axis.
Data is preferably compressed after removing random noise components from the spectrum of the beat signal, thereby ensuring that a target can be detected in a highly reliable fashion without being affected by the noise components and without losing information.
Furthermore, random noise is preferably removed by replacing data, whose original signal strength is lower than a first threshold value and whose difference value is lower than a second threshold value, with a specific value such as zero, thereby ensuring that a target can be detected in a highly reliable fashion without losing information associated with signal components originating from the reflected wave from the target.
Furthermore, depending on conditions, the process of detecting a target is also preferably performed by the radar device, and either the resultant detection data or compressed data is selectively transmitted to the host device. This makes it possible to transmit data in a proper form depending on the bandwidth of the transmission line or the processing capability of the host device.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments with reference to the attached drawings.